Silica-filled rubber articles are known in the rubber industry. In comparison with prior technology, these rubber articles have various advantages, such as, for example, in the tire sector, inter alia, improved wet skid behavior and lower rolling resistance are provided. Polysulfide silane compounds are used as additives in order to achieve such good characteristics. These compounds act as coupling agents between the filler, silica and the organic polymer and bring about a reduction in the viscosity of the composition during incorporation of the filler.
Various processes for the production of known polysulfide silanes are described in several patent applications. Most methods start from alkoxysilylalkyl halides which are reacted with alkali metal polysulfides produced in various manners.
According to U.S. Pat. No. 5,405,985, an alkali metal polysulfide is produced by reacting alkali metal sulfide with sulfur in an aqueous solution. The polysulfide coupling agents are obtained by reacting the resultant aqueous polysulfide solution with alkoxysilylalkyl halides in a phase-transfer catalyzed system. It is known that both alkoxysilylalkyl halides and the reaction products obtained therefrom are susceptible to hydrolysis. This process accordingly has the disadvantage that the polymers formed after hydrolysis and condensation exhibit no reinforcing action or exhibit a severely reduced reinforcing action when used in a rubber composition.
For this reason, anhydrous systems are used in other known processes. An anhydrous polysulfide may thus, for example, be obtained in an upstream synthesis step from elemental sulfur and elemental sodium (U.S. Pat. No. 4,640,832). Alternative production methods for anhydrous alkali metal polysulfides start from elemental sulfur and alcohols (U.S. Pat. No. 5,399,739) or from alkali metal alkoxides, hydrogen sulfide and sulfur (U.S. Pat. No. 5,596,116). Another variant uses alkali metal hydrogen sulfide and sodium alkoxide in combination with sulfur in order to produce the polysulfide (DE 3311340).
These known processes have the disadvantage that costly starting materials such as alkali metals or alkali metal alkoxides or the toxicologically and ecotoxicologically questionable compound hydrogen sulfide must be used in the production of sodium polysulfide.
It is furthermore known to dry commercially available "anhydrous" sodium sulfide and then to react it with sulfur in organic solvents. The hydrous sodium sulfide is dried both by contact drying under reduced pressure (JP 7228588, DE 19755760, EP 361998) and by azeotropic drying methods (DE 19610281, JP 7228588). These known processes have the disadvantage that sodium sulfide has a strong tendency to melt on heating which means that even slight deviation from optimum conditions causes the material being dried to stick to the container walls with consequent losses of valuable product. Dried sodium sulfide moreover has a tendency to ignite spontaneously, such that elaborate safety precautions are required for drying, which make the process uneconomic for industrial application.
U.S. Pat. No. 5,663,396 discloses a process in which the polysulfide is produced in an aqueous solution from sodium hydroxide solution and sulfur. The resultant polysulfide solution is then in turn reacted with alkoxysilylalkyl halides in a phase-transfer catalyzed system. There is a great risk in this procedure too that some of the starting compound or product will be reacted to yield polymers which are ineffective for practical application.
In view of the above-described disadvantages of the prior art, there was a requirement for a process which is straightforward to perform industrially and is simultaneously economically viable for the production of anhydrous alkali metal polysulfides, which may be used for the production of the stated polysulfide silane compounds.